27 Mechanical Circulatory Support Systems
27.1 Extracorporeal Membrane Oxygenation
Principle of ECMO
Extracorporeal membrane oxygenation (ECMO) is a procedure for patients with severe lung and/or heart failure who do not respond to conventional treatment. ECMO makes it possible to temporarily take over gas exchange from the lungs or the pumping function of the heart.
An ECMO system is similar in structure to a cardiopulmonary bypass machine: venous blood is drained from the body, pumped to an oxygenator, brought to a certain temperature in a heat exchanger, and then returned oxygenated to the body.
ECMO is a temporary measure to bridge the time until lung and/or cardiac function recovers or until definitive treatment, such as transplantation, is possible. The duration of the use of ECMO is between one day and several weeks.
In this case, the blood oxygenated in the extracorporeal circulation is returned to the patient’s venous circulation (Fig. 27.1 a). In order to reach the pulmonary and systemic arterial circulation, the oxygenated blood must be pumped by the heart. Adequate cardiac function is thus a prerequisite for venovenous ECMO. This method is therefore used when lung function is considerably impaired, but the heart’s pumping function is still sufficiently good. Typical examples are the meconium aspiration syndrome or the persistent fetal circulation syndrome or ARDS.
In venoarterial ECMO, venous blood is drained from the body (Fig. 27.1 b). The blood oxygenated in the extracorporeal circulation is then pumped into the systemic circulation via a cannula in the aorta, bypassing the heart. The ECMO thus assumes both the lung function (gas exchange in the oxygenator) and the heart’s pumping function. This method is used when the myocardial function is not sufficient to supply both pulmonary and systemic circulation (e.g., after cardiac surgery or fulminant myocarditis). The disadvantage of venoarterial ECMO is that the aorta must be cannulated, which has considerably higher risks (hemorrhage, systemic embolism) than the cannulation of veins only.
ECMO has now been used in over 20,000 neonates around the world. Since ventilation options have been improved (high-frequency ventilation, inhaled NO, surfactant) the number of ECMOs performed for pulmonary indications has decreased but the number of ECMOs for cardiac indications has increased.
ECMO is indicated when there is a serious impairment of lung or cardiac function that is reversible or can be successfully treated within a few days up to a maximum of weeks. The severity of the disease and the mortality risk must be weighed against the risks of ECMO when the indication is made.
Typical indications for ECMO are:
Cardiac surgery patients who cannot be weaned from the cardiopulmonary bypass machine (e.g., due to a severe temporary ventricular function disorder or pulmonary hypertension)
Severe congenital heart defects that need to be stabilized until the definitive operation (e.g., hypoplastic left heart syndrome, total anomalous pulmonary venous connection with pulmonary venous obstruction)
Cardiocirculatory arrest in the hospital requiring resuscitation: while continuing resuscitation, ECMO can be implanted, thus improving the chances of survival
Interim measure until a heart or heart/lung transplant
Lung failure: neonates with severe pulmonary disorders (meconium aspiration, persistent pulmonary hypertension) with an oxygenation index over 0.4 (oxygenation index = mean airway pressure × FiO2/PaO2)
Interim measure until implantation of a cardiac assist device
Contraindications for ECMO are disorders that would entail the serious impairment of quality of life despite successful ECMO therapy. A high-grade cerebral hemorrhage is a contraindication, as is a body weight of neonates/infants less than 1,800 to 2,500 g.
27.1.2 Performing ECMO
In infants and neonates, usually the right neck vessels are cannulated for implanting the ECMO cannulas: the jugular vein for a venovenous ECMO and the jugular vein and carotid artery for a venoarterial ECMO. For a venovenous ECMO, blood drainage and return occur jointly via the jugular vein in a double-lumen catheter or separately via the additional cannulation of the femoral vein. If venoarterial ECMO is begun in cardiac surgery, the right atrium and ascending aorta are usually cannulated.
As preparation, the ECMO system is filled with a priming solution (such as Ringer solution, human albumin).
For a venoarterial ECMO, the cardiac output is rapidly taken over almost completely by the ECMO system when it is put into operation. A standard value for cardiac output in childhood is 100 to 150 mL/kg/min (ca. 200 mL/kg/min for neonates). The ECMO flow can be controlled via central venous oxygen saturation, for which a normal level of 70 to 75% is targeted. For a venoarterial ECMO, arterial oxygen saturation should also reach normal levels (> 95%).
Either roller pumps or centrifugal pumps can be used for the extracorporeal circulation. In a centrifugal pump, a kind of propeller (like in a turbine) ensures continuous blood flow. The blood pressure amplitude is eliminated when cardiac output is completely taken over by the ECMO.
For ECMO that is initiated due to pulmonary failure, an attempt is made to eliminate damaging effects on the lungs. Therefore, high ventilation pressure and high oxygen concentration (oxygen toxicity) are avoided. In some cases, application of surfactant may have a positive effect on recovery of the lungs.
The oxygen content of the blood can be increased by either increasing the blood flow through the oxygenator or by increasing the oxygen content of the gas that flows through the oxygenator (sweep gas). Carbon dioxide is exchanged almost exclusively by the flow of sweep gas. Increasing this flow leads to greater elimination of carbon dioxide.
The goal is to set ventilation so carefully that the lungs can regenerate, while preventing atelectasis. To do this, a low respiratory rate (e.g., 5/min), a long inspiration time (e.g., 2 s), limit to peak pressure (e.g., 15–20 mmHg, PEEP around 10–15 mmHg), and low oxygen supply (e.g., FiO2 0.3) are used. When lung function improves, the flow of sweep gas in the ECMO can be reduced.
Heparin is used for anticoagulation. The heparin effect is checked using the activated clotting time (ACT), which can be determined quickly in a bedside test. An ACT of around 180 s is usually targeted.
As a result of the contact with the foreign body surfaces of the ECMO system, there is also a drop in platelets. Platelet levels of 50,000/μL should be maintained.
Hemofiltration can be readily integrated into the ECMO system. It is frequently needed for ECMO patients, as the high volume demand as a result of SIRS under ECMO therapy often leads to serious edema. In addition, patients occasionally have kidney failure with oligouria or anuria due to impaired renal perfusion and hypoxia even before the ECMO implantation.
The weaning process from the ECMO begins as organ function increasingly recovers. The cardiac output pumped by the ECMO is gradually reduced. Catecholamines are almost always needed for cardiac support. Parallel to reducing the ECMO, mechanical ventilation is again intensified, as oxygenation must now be assumed mainly by the lungs. In some systems, it is possible to “short circuit” the extracorporeal circulation on a trial basis by connecting the two cannulas so the blood can be diverted past the ECMO pump and oxygenator to flow back to the body.
If the neck vessels were cannulated, the jugular vein is usually ligated when the ECMO cannulas are removed, while an attempt is made to reconstruct the carotid artery.
There is thus far no consensus as to when ECMO should be discontinued if the organ fails to recover or the prognosis is unfavorable. The decision must be made on a case-by-case basis.